Local Area Networks (LANs) provide a communication facility for data exchange among devices within a moderately sized geographical area. A typical network includes a number of host computers, a number of workstations and other LAN compatible devices arranged in the proximity of each other for use in office automation, distributed data processing and other situations requiring connection to a common local communication medium. It is, of course, desired that the network operate in an economical fashion with the ability for carrying high system data throughput, while accommodating bursty traffic at high peak data rates.
Terminal server devices provide connection between terminals which operate asynchronously to transmit data to and receive data from the LAN. A terminal server typically is dedicated to a plurality of terminals via serial data lines originating from the terminal server and terminating at respective terminals. The terminal server also provides a node for connection to the LAN. The LAN provides for packet data transmission between the terminal server and the host computer. In this way, communication is established between a number of terminals and host computers.
Previously proposed terminal server configurations typically employ a powerful central processing unit (CPU), memory, a LAN interface device having a direct memory access (DMA) controller, and a plurality of asynchronous receiver/transmitter devices, each dedicated to respective terminals linked to the terminal server device.
The terminal server performs data management and movement as follows with this type of architecture. For transmission of data from the LAN to a selected terminal, the LAN interface DMA controller directly accesses the terminal server memory and moves incoming packets of data to memory. The CPU thereafter executes appropriate instructions to transfer asynchronous data, one character at a time, from memory to a selected asynchronous transmitter/receiver which, in turn, supplies the data to the desired terminal. When data is transmitted from a terminal to the LAN, the CPU executes instructions for moving data received from the asynchronous transmitter/receiver, one character at a time, to a location in memory. The LAN interface DMA controller thereafter moves data packets from memory to the LAN. In this way, the host computer receives data from the sender terminal. The terminal server CPU, in addition, handles all protocol concerns, memory management, and user interface.
While these terminal server arrangements perform satisfactorily in many instances, a number of practical problems frequently arise. Principal among these is cost for the implementation of necessary components in the terminal server device. Most of the CPU clock cycles are dedicated to execution of data movement inasmuch as the primary function of any terminal server is to pass data between respective terminals and the LAN. Accordingly, powerful processors and supporting logic are required. For example, many known terminal servers require 16-bit 68000 family processors to adequately support the data movement.